1. Field of the Invention
The present invention generally relates to a method and apparatus for introducing vibrational energy into plastic concrete structures in successive stages or increments. More particularly, the present invention relates to a method and apparatus for affecting the firmness profile of a concrete structure by introducing vibrational energy at the resonant frequency of the wet concrete into said structure while it is in a plastic state during its placement.
2. Description of the Prior Art
In constructing concrete structures, such as concrete slabs and the like, certain conventional procedures involve simply placing the concrete mass in a form and finishing the top surface in various well known manners and permitting the concrete to harden with no vibration whatsoever. Other procedures involve the use of vibrators placed temporarily into or upon the concrete mass at various locations, with the surface being finished by using various combinations of striking off the surface and/or trowelling operations, including the use of hand trowels, powered rotary trowels and the like.
A problem with prior methods of placing concrete using vibrators is associated with the lack of control of the vibrators. When any one section of a poured concrete slab is vibrated too much, it causes "hard spots" in the concrete slab approximately at the location of the contact with the vibrator. In addition, over-vibration of the concrete can also cause aggregate separation in the vicinity of the vibrator. Aggregate separation and "hard spots" both result in a non-uniform and weakened final slab. For these reasons, prior concrete placing operations typically cautiously "under-vibrate" the concrete mass or may not vibrate the concrete mass at all.
The principal purpose of vibrating plastic concrete in this context is to expeditiously consolidate the concrete mass at as nearly a uniform density as possible by encouraging and assisting the upward migration of water and air which would otherwise migrate slowly or not at all. Entrapment of air and water weakens the concrete, and the slow migration of these materials extends the time required to place and finish the concrete mass.
Prior procedures for the application of vibrations to the concrete mass provide virtually no means to control or to modify the vibrational characteristics (such as frequency, amplitude, etc.) of the vibrators (other than by manually turning the vibrator off and on), and only crude means to control or modify the length of time the vibrators act upon the concrete mass. Therefore, the prior procedures produce a concrete mass in which the degree of consolidation varies from one location to the other (resulting in a structure of inconsistent structural integrity), and in which the time required for water to evaporate from the surface varies from one location to the other (making it very difficult to finish the structure by using automatic or robotic finishing equipment).
Another phenomenon associated with natural (i.e. non-vibrated) consolidation and curing of concrete is the entrapment of moisture inside of the curing mass. As poured, concrete mixtures commonly comprise an amount of water far exceeding the quantity which is actually necessary to effect proper curing and maximum strength of the concrete mass. The excess water is intentionally added to the concrete mixture in order to facilitate transporting, pouring, forming, and finishing operations. If left stagnant (i.e. un-vibrated), pressure from the weight of the concrete mass initially slowly presses some of the excess water upward through the concrete mass, thus initially inducing migration of some of the excess water towards the surface of the slab and, at the same time, effecting the consolidation of the concrete mass near the bottom of the slab. The concrete begins to cure, even while the concrete mass may not yet be optimally consolidated. This curing of the concrete mass retards the migration of the remainder of the excess water towards the surface of the slab. At the same time, in many instances (particularly when the slab is poured in sunlight, on a windy day of low humidity), water may evaporate so quickly from the top surface of the slab that the concrete at the top prematurely dries out and begins to cure. This results in the setting of the concrete at or near the upper surface of the slab, which further retards migration of excessive water from the concrete mass below to the surface. Ultimately, this phenomenon results in the entrapment of the moisture inside of the concrete slab. Over time the moisture bubbles dry out, leaving small air pockets throughout the solid concrete slab. Such air pockets reduce the final strength of the concrete slab.
In prior concrete slab placing operations, de-watering techniques are sometimes used wherein the concrete mass is poured and formed into a structure having an upper surface, and the mass is then de-watered by applying a vacuum water extracting system over the wet concrete surface. Alternatively, the surface of the concrete mass is de-watered by placing absorbent material (such as burlap or the like) over the wet concrete surface, and then spreading a desiccant (such as dry cement) on the burlap. After the de-watering process has been completed the burlap or the vacuum water extraction system is removed. The surface is then conventionally finished. Prior concrete finishing procedures are labor intensive and require extensive use of skilled labor and considerable time expenditure in properly carrying them out.
The following U.S. Patents relate to developments in the introduction of vibration into a concrete mass by the use of vibrating devices that are immersed in or otherwise associated with the concrete mass: U.S. Pat. Nos. 2,015,217; 2,223,734; 2,269,109; 2,293,962; and 2,332,687.
While the above patents relate to the vibration of a concrete mass, none of them suggests a staged or step-by-step vibration of a concrete slab, deck or the like in which each stage of the vibration introduced onto or into the concrete mass affects the mass of concrete to specific desired depths in the concrete mass; nor do any of them describe a means of determining the depth to which the concrete mass has been consolidated or should be vibrated.
Co-pending U.S. patent application Ser. No. 08/055,004 discloses a method and apparatus for applying staged vibration to plastic concrete structures. It is desirable, when employing such staged vibration methods and apparatus, to minimize the amount of vibrational energy which must be imparted into the concrete structure in order to cause the expeditious consolidation of the concrete mass. It is also desirable, when employing such staged vibration methods and apparatus, to minimize the vibrational energy which is imparted into the already sufficiently-consolidated portion of the concrete mass. It is also desirable, when employing such staged vibration methods and apparatus, to simplify the construction, manufacture, and use of the vibrating and sensing equipment.